Aluminum spent potliner (SPL) is generated from the Hall electrolytic cells used in the reduction of Bayer alumina (Al2O3) to metallic aluminum. The cell, or pot, is lined with carbon, backed by thermal insulation, with a metal collector plate buried in the carbon lining to conduct the current. Thus, the entire carbon lining acts as the cathode. Reaction of the carbon with air at elevated temperatures results in the formation of several cyanide (CN) compounds, primarily sodium cyanide. The primary toxic constituents include the cyanide compounds, at concentrations up to 0.2 weight percent (wt pct), and fluorine, at concentrations up to 15 wt pct. Spent potliner is recovered from the reduction cells after their useful life, and includes the original carbon lining, along with some portion of the insulating refractory used to enclose the cell. The resulting SPL contains carbon, at concentrations from 12-50 wt pct, inorganic oxides (slag formers), at concentrations 30-60 wt pct, and the before mentioned toxic constituents. Due to its toxic constituents, SPL has been listed as a hazardous waste, designated K088, by the U.S. Environmental Protection Agency (EPA). No approved treatment process currently exists for SPL, due to recent legal decisions, and a land ban enacted by the EPA has stopped all landfill. Annual generation of SPL from U.S. plants is estimated at 2-300,000 tons, which is currently stored above ground either on-site or at approved hazardous waste repositories. The SPL thus represents a major future expense and liability to the industry.
The only approved treatment technology for SPL, used by Reynolds Metals Aluminum Co., mixes the SPL with sand, incinerates it, and disposes of the residue in a dedicated landfill in Gum Springs, Ark. However, other aluminum smelting companies are critical of the process, because it results in an increased volume for landfill, the transportation costs for shipping the SPL to the treatment plant in Arkansas exceed the treatment costs themselves, and the treatment standard adopted by the EPA, the Toxicity Characteristic Leaching Procedure (TCLP), does not accurately portray the effectiveness of the Reynolds"" process to render the SPL nonhazardous. In fact, the waste treatment standard adopted by the EPA for SPL destined for land disposal is being revised.
The inventive process constitutes the use of conventional graphite electrode arc furnace (EAF) technology to simultaneously utilize and detoxify the SPL. Hazardous constituents of the SPL, such as cyanogens, fluorides, and organics, are either a) destroyed during processing, b) converted to valuable compounds and captured within the system, or c) utilized for reduction of iron oxide additives to produce value-added products. The EAF system employed includes a close-coupled thermal oxidizer and state-of-the-art air pollution control system (APCS). A thermal oxidizer and components of an APCS are necessary for the success of the inventive process.
Detailed chemical analyses of typical SPL samples has shown that several cyanide species are generally present, along with 5-15% fluoride and 15-50% carbon. The remainder of the SPL consists of refractory brick from the reduction cell, composed of inorganic oxides and generally very high in alumina (Al2O3). The complex nature of the SPL, and the process economics, have stymied the development of an industry-wide treatment process. However, it is specifically this complex nature that makes the EAF the ideal vessel for the processing and ultimate recycling of SPL.
The invention consists of certain novel features and a combination of parts hereinafter fully described, and particularly pointed out in the appended claims, it being understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantages of the present invention.